According to what I can find "Heart Calm" has several different formulas, mainly potassium and magnesium.
If you are low on these minerals a supplement is certainly beneficial, especially if you are on a loop diuretic such as Furosemide or Torsemide, however if you are not low on magnesium excessive amounts of said mineral generally just cause diarrhea if taken orally, (that's through the mouth). Low Potassium can cause heart and other disorders, but with Potassium an overdose can also cause heart and other disorders as well.
Your best bet is to get a blood test to see if your Potassium or Magnesium levels are within tolerances. In many cases a vitamin or mineral overdose is more dangerous than a not enough, although the health food industry is still allowed to push these mega doses of this and that, in many cases these high doses are doing more damage than good.
Your probably better off to stay away from the supplements unless you have a definite deficiency for whatever reason.
Cheers!